In the past sodium has been employed as a modifying agent for the aforesaid aluminium-silicon casting alloys. More recently strontium has been proposed instead of sodium because it gives a more permanent modifying effect than sodium. Such master alloys employing strontium are disclosed, for example, in British Patent Specification No. 1,514,503 which describes a strontium-silicon-aluminium master alloy and in U.S. Pat. No. 3,567,429 which describes master alloys containing strontium and/or barium for use as refining alloys for aluminium silicon alloys.
Strontium has hitherto generally been added to the aluminium-silicon melt in the furnace, either as metal or as a master alloy. Strontium oxidizes at normal aluminium casting temperatures, resulting in the need for relatively high additions of the metal which is relatively expensive. Moreover there is the likelihood of furnace contamination, and hydrogen pick-up in the furnace, occurring. Recently, attempts have been made to overcome these problems by introducing strontium as a low concentration master alloy in rod form into the launder immediately prior to casting. The main disadvantage of this method has been the relatively high addition rates which are required and this adds to the expense of the process. It has been found that higher concentration. Al-Sr master alloys in rod form (5-10% Sr) take 10-15 minutes to dissolve and therefore cannot be used as launder feeds.
The aluminium-silicon alloys are generally also treated with an aluminium-titanium boron alloy in either rod or ingot form for alloy grain refining the as-cast structure giving reductions in micro porosity, improved mechanical properties and improved surface finish. If both the aluminium-strontium containing master alloy and the aluminium-titanium-boron alloy are introduced in rod form then two rod feeding systems are required. Moreover the producton of the aforesaid strontium-containing master alloys in rod form has certain disadvantages, principally that segregation of the strontium phase tends to occur in the extruded billet, greatly limiting the concentration of strontium which can be used.